1. Field of the Invention
The present invention relates to sealing members for polycrystalline ceramic bodies and, more particularly, to sealing member compositions for high pressure discharge lamps composed of alumina.
2. Description of the Prior Art
Electrical discharge devices, such as high pressure sodium (HPS) vapor arc lamps, commonly utilize transparent or translucent high temperature refractory tubes composed of alumina or yttria. Alumina is a preferred lamp envelope material in current commercial practice. Within the alumina tube, an arc discharge extends between two tungsten electrodes to which current is conducted by a hermetically sealed feedthrough assembly.
Because alumina and niobium metal have nearly equal thermal coefficients of expansion, a niobium tube or a niobium wire is typically used in HPS lamps to conduct electrical current through the ends of the alumina arc tube. Tungsten electrodes are welded to the niobium tube. Generally, the niobium feedthrough tube extends through an axial opening in sealing members, which are hermetically sealed to and close each end of the alumina arc tube. The niobium feedthrough is hermetically sealed to the sealing member by a frit or braze seal. Thus, the feedthrough assembly not only seals the discharge tube but also conducts electrical current through the end of the alumina arc tube.
The interface between the niobium metal feedthrough and the alumina sealing member is typically filled with a ceramic, meltable sealing frit, which forms a hermetic seal. Several types of sealing frits have been disclosed. For example, a ceramic frit consisting of Al.sub.2 O.sub.3 -CaO-MgO-BaO is described by Ross in U.S. Pat. No. 3,281,309; and a composition of calcia, magnesia, and alumina is described by Sarver et al. in U.S. Pat. No. 3,441,421. Under normal frit processing conditions however, the feedthroughs exhibit grain growth and recrystalization which affect their ductility, thereby causing premature cracking and limiting lamp life.
Another method to hermetically seal the feedthrough within the sealing member, on a production basis, includes brazing with eutectic metal alloys. These seals are described by Rigden et al. in U.S. Pat. No. 3,428,846 and by Rigden in U.S. Pat. No. 4,004,173, wherein alloys of niobium, titanium, vanadium, and zirconium are utilized. These seals are no longer favored, however, due to long-term embrittlement of the niobium feedthroughs caused by processing temperatures and atmospheres.
Both of the above sealing methods limit the arc tube end temperature (commonly called the "cold spot temperature") to 800.degree. C., due to the softening temperature of the sealing frits or sealing alloys. Also, sodium or other lamp fill material may react with the sealing frit or alloy, thereby limiting lamp life. The latter reaction rate would naturally increase with any increase in the cold spot temperature. Eliminating the frit in the seal would prevent this type of life-limiting reaction and allow for higher cold spot temperatures, thereby permitting altogether new low vapor pressure lamp chemistries.
A direct niobium-to-ceramic sealed lamp is described in U.S. Pat. No. 4,545,799 to Rhodes et al, which is incorporated herein by reference. The assembled sealing members, or inserts, and arc tube are partially sintered, the niobium feedthroughs are inserted into the axial openings in the inserts, and the assembly is fully sintered to translucency, forming a seal as the insert material shrinks during the sintering process.
A similar sealing method is described in U.S. Pat. No. 5,057,048 to Feuersanger et al., which refers to other alumina- or yttria-based materials, suitable for lamp fabrication, in the sealing members that allow the lamp assembly to have a pinched-off, ductile feedthrough as well as providing for a sealing process for achieving the required ductility at the outer end of the feedthrough.
Sintering aids, such as yttrium oxide and magnesium oxide, are disclosed in U.S. Pat. No. 3,711,585 to Muta et al. and U.S. Pat. Nos. 4,762,655 and 4,797,238 to Rhodes et al. for producing translucent, light-transmitting Al.sub.2 O.sub.3 -based lamp tubes. These additives enhance the sintering rate, and inhibit pore-entrapping grain growth in sintering of Al.sub.2 O.sub.3 to translucency. These additives, however, react with Al.sub.2 O.sub.3 at high temperatures to form second phases, Y.sub.3 Al.sub.5 O.sub.12 and MgAl.sub.2 O.sub.4. These phases scatter light when they form in a Al.sub.2 O.sub.3 body. Consequently, only minimal concentrations of the additives necessary to enhance sintering can be employed in the production of translucent Al.sub.2 O.sub.3 arc tubes. Higher concentrations inhibit light transmission, thereby degrading an essential characteristic of the Al.sub.2 O.sub.3 arc tube.
Arc tubes for arc discharge lamps, such as the HPS lamp, are often fabricated with an internal washer or insert. The insert, nominally the same composition as the arc tube, is placed inside the tube at each end in the unfired, or green, state. The starting powder particle size or prefiring of the insert is adjusted such that the arc tube shrinks onto and over the insert with 3-10% interference. This differential shrinkage forces the two components together to form a bond and to seal the parts together by solid state sintering. The classic hermetic seal required for long-life lamps is formed by adding to the arc tube and insert a second external Al.sub.2 O.sub.3 washer of the same composition as the arc tube. A ceramic sealing frit is used to seal the washer and the insert to the electrode assembly. This construction appears to operate satisfactorily in lamps that require only low cold spot or end temperatures of about 700.degree. C., but is expensive due to the extra washer and sealing frit. The mercury-sodium amalgam temperature for maximum efficacy is within the range from about 615.degree. C. to 750.degree. C. depending on the sodium to mercury ratio (amalgam ratio) and the arc tube end diameter (cold spot).
Another possible lamp construction scheme eliminates the external, or top washer. This scheme is less expensive and therefore highly desirable. The major difficulty is that, without the use of the external washer/frit combination, the insert must form a hermetic seal with the arc tube wall that will last the life of the lamp, typically 24,000 hours. This requires the utmost control of the smoothness of the inside diameter of the arc tube and the outside diameter of the insert, so that no through grooves or voids are retained after sintering. Further, the shrinkage rates of the components must be controlled carefully to insure good bonding between components. If too low an interference fit is experienced, bonding is incomplete and hermeticity is not achieved. No void should extend over 1/3 of the insert's thickness. Conversely, if too high an interference fit is experienced, the strain associated with the interference fit causes cracking of the arc tube. Thus, the conditions for a successful hermetic seal are difficult to achieve and, if not met, can result in a higher than acceptable reject rate, or premature failure.
It is therefore an object of the present invention to provide an alumina arc tube sealing member composition which permits fabrication of electrical discharge lamps including a cap seal or a hermetic insert washer with or without the use of sealing frits or brazing alloys. It is a further object of the present invention to provide an alumina arc tube sealing member composition which permits formation of sintered hermetic seals to a green, a prefired or a fully sintered translucent alumina arc tube simplifying the subsequent frit sealing of niobium feedthrough assemblies into highly reliable lamps.